For an engine to which fuel is supplied by a fuel injection device, fuel needs to be supplied to an injector with sufficient pressure at the start of the engine in order to inject fuel in an amount required at the start of the engine to improve startability of the engine. Thus, as disclosed in Japanese Patent Laid-Open No. 2005-23911, a control device that controls an engine to which fuel is supplied by a fuel injection device includes means for first driving a fuel pump for a predetermined time when the control device is powered on, and the pressure of the fuel supplied to an injector is increased to a predetermined value before the start of cranking of the engine.
When a starting device starts the cranking of the engine, the control device controls fuel injection timing, fuel injection time, and ignition timing based on information obtained from various sensors mounted to the engine.
In the conventional control device, crank angle information of the engine is obtained from an output of a crank angle sensor mounted to the engine, and first fuel injection is performed when the fuel injection timing is detected based on the crank angle information, thereby inevitably causing a delay between the start of the cranking of the engine and the first fuel injection. If the first fuel injection delays, an air/fuel ratio of an air/fuel mixture supplied into a combustion chamber of the engine reaches a predetermined value with a delay, thereby reducing startability of the engine.
Poor startability of the engine increases time for driving a starter motor to increase power consumption of a battery, which requires a high capacity battery and is uneconomical.
When the starter motor is driven, an extremely large amount of electric power is consumed to significantly reduce a terminal voltage of the battery. FIG. 11 shows an example of measurement results of changes in rotational speed and battery voltage at the start of the engine, with driving time of the starter motor on the axis of abscissa. In FIG. 11, the curve a indicates a rotational speed (a cranking speed) of the engine, and the curve b indicates a battery voltage. When the battery voltage decreases, a driving voltage of the injector decreases, and thus a valve of the injector is opened with a delay to prevent a desired amount of fuel from being injected from the injector.
Generally, in a fuel injection device for an engine, the pressure of fuel (fuel pressure) supplied to an injector is controlled to be maintained constant by a pressure regulator, and the amount of fuel injected by the injector depends on time for opening a valve of the injector (valve opening time). The injector does not open the valve immediately after a driving voltage is supplied, but there is delay time (referred to as ineffective injection time) between when the driving voltage is supplied and when the valve is actually opened. Thus, when the fuel injection amount is controlled, valve opening time (time for opening the valve of the injector) required for obtaining the fuel injection amount required for maintaining an air/fuel ratio of an air/fuel mixture within a predetermined range is arithmetically operated relative to various control conditions as effective injection time. Then, the ineffective injection time plus the effective injection time is regarded as apparent fuel injection time, and an injection command signal having a signal width corresponding to the apparent fuel injection time is provided to an injector driving portion. The injector driving portion supplies the driving voltage to the injector while receiving the injection command signal, and injects fuel from the injector during the effective injection time.
FIG. 10 shows the relationship between an injector driving voltage and the ineffective injection time. As shown in FIG. 10, the ineffective injection time is increased with decreasing injector driving voltage. Thus, as shown in FIG. 11, if the battery voltage decreases at the start of the engine to reduce the injector driving voltage, the ineffective injection time of the injector is increased to delay opening of the valve of the injector. The delay of the opening of the valve of the injector causes a shortage in fuel injection amount even if the fuel pressure supplied to the injector is sufficiently increased, thereby inevitably reducing startability of the engine.
Thus, it is considered that the battery voltage is detected, and the ineffective injection time added to the effective injection time is corrected according to the battery voltage, thereby preventing a reduction in the injection amount with decreasing injector driving voltage.
However, at the start of the engine, changes in internal pressure of a cylinder caused by a stroke change of the engine cause a load applied to the starter motor to vary, and as indicated by the curve a in FIG. 11, the rotational speed finely changes according to crank angles, and the variation in the load causes the battery voltage to change. Further, switching of energization performed by a commutation mechanism constituted by a commutator and a brush (for a brushless motor, switching of energization patters) causes a driving current of the starter motor to finely vary, and thus a waveform of the battery voltage significantly changes as shown in FIG. 11.
As described above, the battery voltage significantly changes at the start of the engine, and thus it is difficult to detect the battery voltage to precisely arithmetically operate the ineffective injection time, and difficult to properly correct the ineffective injection time relative to the battery voltage to control the fuel injection amount with high accuracy.
In order to prevent a shortage in fuel injection amount at the start of the engine, a microfilm of Japanese Utility Model Laid-Open No. 60-90540 proposes that an injector is driven to perform first fuel injection when a key switch is closed, and then a starter motor is activated.
If sufficient fuel pressure is supplied to the injector when the key switch is closed, and the starter motor can be activated after the first fuel injection without a delay, the control described in Japanese Utility Model Laid-Open No. 60-90540 can prevent a shortage of fuel at the start of an engine to improve startability of the engine.
However, when the key switch is closed, the fuel pressure supplied to the injector is often insufficient. If the fuel pressure supplied to the injector is insufficient when the key switch is closed, a desired amount of fuel cannot be injected even if first fuel is injected when the key switch is closed, thereby failing in ignition of the fuel at the time of first ignition thereafter, and inevitably reducing startability. The failure in the ignition of the fuel causes unburned gas to be exhausted to pollute the atmosphere.
Further, if time is long between when the key switch is closed and when the starter switch is closed, the fuel injected by the injector adheres to an inner surface of an intake pipe or an inner surface of a cylinder to form a liquid film, which causes a shortage of fuel that contributes to combustion and reduce startability of the engine. Also in this case, the failure in the ignition in the cylinder causes unburned gas to be exhausted to unpreferably pollute the atmosphere.